1. Field of the Invention
The present invention relates to hip arthroplasty. Specifically, the present invention relates to a prosthetic cementless femoral stem for use in hip replacement strategy.
The hip joint is one of the most important joints in the body, as it is responsible both for supporting the weight of the body and balance. Hip replacement surgery may be necessary when there is avascular necrosis of the femoral head, oseteoarthritis of the hip joint, or a substantial hip fracture, among other reasons. Depending upon the specific issue, one of two types of hip replacement surgery will be used. A hemiarthroplasty is the replacement of the femoral head only. A full hip replacement entails the replacement of both the femoral head and the acetabulum. In either case, a stem prosthesis for the femoral head that is biocompatible, has good mechanical properties, and smoothly interacts with the acetabular articulating surface is necessary.
Several types of femoral head prostheses are currently available in the prior art. Stem prostheses are currently the most common type of femoral head prosthesis, but stemless variations are also available. A femoral stem prosthesis consists of an artificial femoral head that is connecyed to a body portion and that articulates with the acetabulum or the artificial acetabular surface. A femoral stem prosthesis is installed by removing the head and neck portions from the patient's femur and then securing the stem prosthesis within the hollow interior of the femur. Stemless femoral prostheses affix directly to the femoral head, rather than providing a new femoral head. Although less invasive, stemless femoral head prostheses are initially less stable and therefore require longer recovery periods.
Femoral head stem prostheses also come in cemented or cementless variations. Cemented hip stems are secured within the hollow interior of the femur by bone cement. Cementless hip stems are specifically fitted within the femur and often use various techniques to encourage osseointegration with the prosthetic components to secure the device. Results between cemented and cementless hip stems are generally similar if the patient has good enough bone quality to accept a cementless prosthesis, but cement can degrade over time, so cemented stems are generally not used in younger patients.
Currently available hip stem prostheses have a variety of problems. Currently available hip stem prostheses alter the way the body load is distributed through the femur. The body load with a natural, healthy femur is distributed through the femoral head, down the neck, and then downwards through the cortical bone of the femur. The body load with a hip stem prosthesis is distributed from the head, down the neck, and instead then laterally into the cancellous bone because the stem of the prosthesis extends into the femoral canal. Since the hip stem prosthesis changes the normal load distribution in the femur, it can lead to bone resorption and loosening of the prosthesis because bone remodels in response to applied stresses. The process, called stress shielding, occurs when biomechanical forces are transferred to the distal regions of the prosthesis, causing bone to be resorbed in the more proximal portions of the bone near the neck of the prosthesis since the stresses normally present in those portions that stimulate bone growth have been removed by the presence of the prosthesis. When bone is resorbed near the prosthesis neck, the entire prosthesis is no longer secured firmly within the femoral canal and, in serious cases, can result in periprosthetic fracture.
The present invention addresses these issues in multiple ways. First, the cementless hip stem has a collar that directly transfers load to the cortical bone below the neck, reducing the potential for bone resorption in the region of the bone proximal to the femoral head. Second, the cementless hip stem's distal shank end is divided into two endosteal plates that are adjustable to ensure an ideal fit within all types of femurs. Third, the endosteal plates are secured to the cortical bone via a plurality of screws. This helps transfer the body load from the distal end of the prosthesis to the cortical bone, rather than to the cancellous bone. Lastly, the medial endosteal plate has a plurality of spurs that engage with the cancellous bone within the femoral canal to help secure the prosthesis and further distribute the body load. In this regard, the present invention fulfills a substantial need within the art for a cementless hip stem that performs better than the currently available models.
2. Description of the Prior Art
Devices have been disclosed in the prior art that relate to prostheses for use in arthroplasty. These include devices that have been patented and published in patent application publications. These devices generally relate to cemented and cementless hip stem prostheses. The following is a list of devices deemed most relevant to the present disclosure, which are herein described for the purposes of highlighting and differentiating the unique aspects of the present invention, and further highlighting the drawbacks existing in the prior art.
One such device is U.S. Pat. No. 4,919,673 to Willert, which discloses a femoral head prosthesis with a centering rod for guiding the stem during introduction of the prosthesis into the femoral canal. The centering rod is aligned on the central axis of the femoral canal, the bone cement is poured in, and then the hip stem prosthesis is threaded onto the centering rod and guided into the femoral canal, ensuring proper alignment. This femoral head prosthesis is cemented and thus not secured in the same way as the present invention. Furthermore, this femoral head prosthesis lacks all of the load distribution mechanics that the present invention utilizes, including the collar and the pair of adjustable endosteal plates. Therefore, this invention suffers from all of the problems commonly associated with such implants, such as stress shielding.
Another such device is U.S. Pat. No. 4,921,501 to Giacometti, which discloses a prosthetic femoral stem head having a longitudinally-aligned slot beginning at the distal end of the stem and extending upwards towards the proximal end of the prosthesis at an angle. The distal end of Giacometti has increased resilience, while also being non-resilient relative to lateral loading from the femoral head. This non-resilience to lateral loading is ideally suited for re-operation procedures, which generally extend new prostheses into the increasingly narrow portion of the femoral canal. One embodiment of the present invention is specifically designed for re-operation procedures, like Giacometti, but it has additional elements designed to assist with the displacement of the body load that Giacometti lacks, such as a collar and adjustable endosteal plates.
Yet another such device is U.S. Pat. No. 5,156,627 to Amstutz, which discloses a collarless femoral hip prosthesis. Amstutz describes a series of eight femoral head prostheses that are configured to fit tightly with a wide range of femurs. Although Amstutz seeks to provide increased compatibility with patients of different builds, it nonetheless is not as highly compatible as the present invention because the present invention has a pair of endosteal plates that are freely adjustable and are not limited to merely eight configurations.
U.S. Pat. No. 5,725,595 to Gustilo discloses a cannulated cementless hip stem prosthesis. This invention has three main features. First, a guide rod can be inserted through the cannulated stem portion to precisely align the stem within the femoral canal. Second, Gustilo has a flexible distal portion that deflects loads away from the prosthesis and into the natural bone. Lastly, Gustilo has a roughened interstitial surface for increased interaction with the living femoral bone. The distal portion of the present invention is not flexible, but rather is freely adjustable and transfers load to the cortical bone by being engaged to said bone with screws.
U.S. Pat. No. 6,007,581 to Noble discloses a cementless asymmetric hip stem for hip replacement procedures. Noble's stem has a twist in the mid-stem region, permitting an improved fit while at the same time minimizing enlargement of the femoral canal. Noble further has a slot on the distal end that reduces bending stiffness, like Giacometti. The present invention has an open, separated section at the distal end of the femoral stem prosthesis as well, but this separation is not created to increase flexibility of the distal region. The separated endosteal plates at the distal end of the present invention instead may be adjusted depending upon the width of the individual patient's femur and are not designed to increase flexibility per se.
U.S. Pat. No. 7,947,135 to Fonte discloses a femoral stem prosthesis with a bone-locking mechanism comprised of shape memory material. The shape memory material is deposited within the femoral canal, the hip stem is then placed into position, and then an induced temperature change causes the shape memory material to expand and lock the hip stem prosthesis in place. The bone-locking mechanism is intended to increase primary stability, which leads to a reduction in patient recovery time and allows for greater weight-bearing during recovery. The present invention is locked into place using a variety of means, most notably the cortical screws secured through the endosteal plates, but does not use shape memory material to accomplish the goal of increased stability.
Finally, U.S. Pat. No. 8,252,062 to Bandoh discloses an artificial cementless hip prosthetic stem having an outer body and an inner stem construct. The outer body is a smooth, substantially flat surface that fits within the femoral canal. The inner stem construct then fits within the outer body and an adhesive filler is applied between the outer body and the inner stem construct. The purpose of fitting the hip stem prosthesis within an outer body is to more evenly distribute the forces applied to the prosthesis head throughout the femur, rather than just to the bone in contact with the distal portion of the prosthesis. The present invention has the same goal in more evenly distributing the body load through the cortical bone, but accomplishes this task by directly engaging the endosteal plates with the cortical bone and having a collar that directly transfers load to the cortical bone.
The present invention comprises a new and novel femoral stem prosthesis for use in hip replacement surgery. The cementless hip stem prosthesis has a collar that directly transfers load to the cortical bone near the neck of the device, a plurality of holes in the fixed end of the device to encourage osseointegration with the prosthesis, and a pair of plates secured to the femoral cortical bone that transfer remaining stresses from the distal portion of the prosthesis to the surrounding cortical bone, rather than the cancellous bone. It substantially diverges in design elements from the prior art and consequently it is clear that there is a need in the art for an improvement to existing femoral head prostheses. In this regard the instant invention substantially fulfills these needs.